Process of preparing a helically crimped polypropylene filament



June 11, 1963 a E. MARTIN 3,093,444

PROCESS OF PREPARING A HELICALLY CRIMPED POLYPROPYLENE FILAMENT Filed July 10, 1961 2 Sheets-Sheet 1 INVENTOR BRUCE EUGENE MARTlN BY uxwwmkj ATTORNEY B. E. MARTIN June 11, 1963 PROCESS OF PREPARING A HELICALLY CRIMPED POLYPROPYLENE FILAMENT Filed July 10, 1961 4 2 Sheets-Sheet 2 BRUCE EUGENE MARTI N BY M1 )1 ATT RNEY where L is the length of the yarn under just suflicient tension to straighten out all the crimps and L, is the length of the crimped yarn after the tension is released. For a given filament and helical diameters, the crimp index is proportional to the number of crimps per inch.

By drawing is meant a process wherein a solid filament is positively forwarded by a first moving means (e.g., a feed roll) to a second moving means operating at a higher speed (e.g., a draw roll). The expression draw ratio and designated as X means the ratio:

Speed of second moving means Speed of first moving means By maximum draw ratio is meant the largest possible draw ratio for a given set of drawing conditions that a given filament sample will draw continuously without breaking. As is known, drawing conditions affecting the maximum draw ratio include the speed of drawing, the medium through which or on which the drawing takes place, as gas, liquid, pins, plates, rolls, and the temperature of the drawing medium. Under any one set of drawing conditions, the maximum draw ratio of a filament will depend upon its characteristics such as molecular weight, denier, and previous treatments, including spinning, drawing, heating, etc.

The polypropylene used to make the fibers to be treated according to this invention, may be of any high molecular weight polymer characterized by a melt index (ASTM Standards, 1958 D-l238-57T, Part 9, page 38), of 0.1 to 200. The polymer should preferably show a stifiness of greater than 120,000 p.s.i. when prepared in test bars according to ASTM test D-747.

The expression inherent viscosity as used in the examples is defined as:

wherein c is the concentration in grams (0.10) of the polymer in 100 ml. of the solvent (decahydronaphthalene) and (n) is the relative viscosity which is the ratio of the flow times in a viscosimeter of polymer solution and of the solvent, both at 130 C. An antioxidant (0.2%) is normally added to both the polymer solution and the solvent. The inherent viscosity is indicative of the molecular weight of the polymer.

There is an obvious practical lower limit on the molecular weight of the polymer used in this invention inasmuch as a material limitation in the process of the invention is that the filament be drawn at least 60% (or at least 25% in the modified process of this invention) of the maximum draw ratio. As the molecular weight of the polymer is lowered, the maximum draw ratio increases and lower and lower draw temperatures and/or higher and higher drawing speeds are required to obtain a maximum draw ratio of which a draw of 1.4x to 2.6x amounts to 60% (or 25%). Thus a point is reached where the drawing temperature must be impractically low or the drawing speed impractically high, or both, in order to reduce the maximum draw ratio to a point where a draw of 1.4x to 2.6x will amount to the requisite proportion of the maximum draw ratio according to the teachings of this invention.

The polypropylene filament used as the starting material in this invention is critical. The crystalline structure of the filament must be such that the filament has gamma orientation and an orientation angle of to 36 before, in accordance with this invention, it is drawn at the minimum draw ratio of 1.4x. It is preferred that undrawn filament be used as the starting material but, if it is desired to draw the filament in the process of getting a starting filament of the requisite characteristics, this is permissible. However, when such pre-drawn filament is used, the pre'drawing must be limited to a draw ratio of no more than 1.86X in order to carry out the process of this invention. The reason for this is because predrawing at a draw ratio of l.86 together with drawing in the process of this invention at a draw ratio 1.4x, the minimum permissible in the drawing step of the invention, comes to a total of 2.6x, the maximum total draw ratio permissible in this invention. Likewise, in carrying out the more limited, preferred drawing step of this invention, the pre-drawing must be limited to a draw ratio of 1.2x because the drawing step of the process requires a draw ratio of at least 1.8x and yet the total draw ratio (including pre-drawing) should not exceed 2.2x. A draw ratio of 1.8 on top of the pro-draw ratio of 1.2x comes to just under 2.2x.

Preparation of filaments having the characteristics critical for use as the starting material in the process of this invention, is illustrated in the examples but the invention is not dependent on the method of preparing the filaments so long as they have the critical crystalline structure indicated by gamma orientation and the requisite orientation angle.

As the examples show, suitable filaments for use in this process can be obtained simply by extruding the filaments under proper spinning conditions without involving a drawing step. The spinning variables are adjusted to regulate the viscosity of the polymer melt as extruded through the spinneret hole, and the viscosity of the filament as it changes from the molten to the solid state in the threadline. Under any one set of conditions of polymer type, separate orifice dimensions, number of orifices, and polymer throughput, control is achieved through temperature regulation and windup speed. The temperature will be a function of the temperature of the molten polymer before being extruded, the rate of extrusion, the geometry of the spinneret, the extent of cooling or heating on the spinneret by outside heating or quenching air. A cooling air quench may be used as the filame s form to assist with the temperature control.

However, suitable filaments can also be obtained by spinning filaments having gamma or even only potential gamma orientation and an orientation angle somewhat greater than 36 and treating such filaments so as to impart to them the requisite characteristics for use in this invention. To illustrate, the orientation angle of the filament, if not greater than about 55 may usually be brought into the 10 to 36 range by submitting the filament to a brief heat treatment at C. to 160 C., preferably C. to C. Only a brief heat treatment is required.

If the filament has potential gamma orientation, it usually can be converted to gamma orientation by a separate drawing step and heating the filament at around 130 C. However, as already pointed out, there is a limit of 1.86X on the draw ratio in any preliminary drawing step and, in fact, a limit of 1.2 if the filament is to he used in applicants preferred process. Accordingly, even though a polypropylene filament is considered as having potential gamma orientation if the filament can be converted to one having gamma orientation upon drawing anywhere within the range 1x to 2.5x, it will be apparent that some filaments may be pre-drawn too much for use in the present invention in the preliminary step of converting them to gamma orientation.

In carrying out this invention, it is advantageous in the drawing step to pass the filament around a snubbing surface such as a snubbing pin or like hard surface having a radius of curvature of no more than /2 inch. Usually, the filament is given one wrap around the pin although passing the filament 30 around the snubbing pin is. effective.

In the modified process of this invention, the final crimping effect is enhanced by passing the filament, after drawing, under tension at least 30 around a pin or other hard surface having a radius of curvature less than 0.5 inch. When this additional step is taken, it has been found that the actual draw ratio used in the drawing step can be as low as 25 of the maximum draw ratio and still a satisfactorily crimped filament is obtained. It is preferred, however, to draw the filament at least 50% of the maximum draw ratio.

Reference is made hereinafter to the accompanying drawings wherein:

FIG. 1 is a schematic showing of a filament spinning and treating apparatus suitable for producing filaments for use in this invention;

FIG. 2 is a graph illustrating relationships between draw speed and maximum draw ratio with respect to certain filaments described in Example III;

FIG. 3 is a graphic representation of an X-ray diffraction photograph illustrating gamma-type orientation which characterizes the polypropylene filaments used in this invention; and

FIG. 4 is a graphic representation of an X-ray diffraction photograph illustrating the normal orientation characteristics of polypropylene filaments heretofore known.

The following examples illustrate specific embodiments of the invention.

EXAMPLE I Crystalline polypropylene of melt index 0.7 (inherent viscosity of 2.75) (Profax, made by the Hercules Powder C0. of Wilmington, Delaware), is extruded as a melt at 296 C. through spinneret 1 (FIG. 1) at about 260 C. The spinneret contains 20 round orifices each 0.03 inch in diameter. The filaments thus formed are passed through a cylindrical heater (indicated at A, FIG. 1) 2.75 inches inside diameter and 2 inches long, the top of which is located 1.5 inches below the spinneret 1. The inside of the heater has a temperature of about 360 C. The filaments are then quenched by passing them through a cylin drical quenching chamber (indicated at C, FIGURE 1) 2 inches inside diameter, located 7 inches below the spinneret 1. The double-wall quenching chamber is supplied with 25 C. air at a rate of about 2 cubic feet per minute. The air passes through a porous metal band 1.5 inches high fitted in a slot in the inner wall of the quenching chamber, and impinges on the filaments.

The filaments are then advanced through yarn guide 8 which gathers them into a so-called yarn and then 4 wraps around feed rolls 3 and 4 rotating at 250 yards per minute (y.p.m.) and then 4 wraps around draw rolls 5 and 6 rotating at 550 y.p.m. and, finally, wound on the bobbin 7. Feed roll 4 rotates in an ice-water bath indicated at B, FIG. 1. In this manner, the yarn is drawn 2.2x which is approximately the maximum draw ratio under these conditions.

When the yarn is released from the tension on the bobbin, it spontaneously crimps to at least helical crimps per inch of crimped length. This crimp is regular and quite uniform, the crimp reversal points occurring about three times per inch.

The as-spun yarn in this example, i.e., the yarn before the drawing step, has an orientation angle of about 16, gamma orientation, and a gamma intensity ratio of 2.0. Thus the process of this invention was applied to an undrawn yarn having the requisite characteristics for the starting yarn to be used in the present invention.

While not a part of the process or this invention, the drawn (crimped) yarn above was further improved by heating for 30 minutes in a 130 C. air oven under no tension with a linear shrinkage of about 30% resulting. This hot relaxation improves the recovery from compression deformation of the yarn. The relaxed filaments forming the yarn (12.1 denier/filament) have a tenacity of 2.2 grams per denier and elongation at the break of 500%, an initial modulus of 4.5 grams per denier, a crimp: index of 61% and have at least 25 helical crimps per inch. This crimped yarn is made into excellent tufted carpets.

EXAMPLE II A 20-filament yarn is prepared as in Example I using the same 0.7 melt index polypropylene but with the omission of the heater and quenching chamber. The as-spun yarn, with no drawing, has gamma orientation and an orientation angle of about 14 This yarn is continuously drawn as in Example I at a feed/draw speed of 333/700 y.p.m. (i.e., a 2.1x draw ratio) and the yarn is sprayed with 20 C. water as it leaves the feed roll 4 of FIG. 11. The maximum draw ratio under these conditions is approximately 2.1 X. When the yarn is removed from the bobbin 7, it'spontaneously crimps. The physical characteristics of the yarn are set forth under item A in Table I below.

In a modification of the above, a Aa-inch diameter smooth chromium plated metal pin is placed between the feed roll 4 and the drawing roll 5 so that the yarn is snubbed by the pin and forms an angle of about 360 (one wrap around the pin) with it. The yarn is drawn 1.9x which is its maximum draw ratio under these conditions. The physical characteristics of this yarn are set forth under item B of Table I.

EXAMPLE III This example illustrates the critical nature of the draw ratio range of 1.4x to 2:6 in the important drawing step of this invention. Reference is made to FIG. 2 of the drawings which is a graph setting forth various curves relating draw speed and maximum draw ratio. All yarn used in this example is undrawn at the start and all drawing is done at the maximum draw ratio under the conditions prevailing.

The polypropylene of Example I is spun under the conditions there set forth except for the following departures. The quenching chamber is omitted. The location of the heater is the same but the heater used here comprises a coil of resistance wire around a metal cylinder two inches high and having an inside diameter of 2.75 inches. The surface of the cylinder facing the filaments is kept at 500 C. The yarn is not drawn and is wound up on the bobbin at 560 y.p.m.

The yarn produced as above has gamma orientation and an orientation angle of 14. This yarn which is undrawn and possesses the requisite characteristics for use in this invention, is variously drawn between feed and delivery rolls ivhose speed can be adjusted as desired by the operator. A metal pin (1.0 inch diameter) is locate-d between the feed and delivery rolls and the yarn is drawn over this pin (one 360 wrap) at various speeds with the pin at temperatures of 25 C., 50 C., C., and C. in different runs.

The curves shown in PEG. 2 and identified by the pin temperatures used are based on points determined in this manner. Referring to the curve identified 25 C. PIN, the above yarn is drawn with the pin at 25 C. at a given constant draw speed, i.e., input speed or speed of feed roll (500 y. p.m., for example, was one such given draw speed), and the speed of the delivery roll was increased until the yarn broke, the maximum draw ratio for the yarn under the specific conditions prevailing thus being established at just below the draw ratio at which the yarn broke. The rolls were strung up again with the above yarn which was then drawn at the established maximum draw ratio for the draw speed and a sample taken for examination. Thus a point is determined on the graph (FIG. 2), i.e., at 500 y.p.m. draw speed on the abscissa and 1.4 maximum draw ratio on the ordinate. The same procedure is followed at a plurality of draw speeds (input speed), with the pin at 25 C., and thereby sufiicient points are determined to establish the curve 25 C.

7 PIN on the graph, and a series of samples of yarn drawn at the maximum draw ratio at the difierent draw speeds is collected. In like manner, this same procedure is followed with the pin at temperatures of 50 C., 75 C., and 100 C.

Each sample above collected where the draw ratio was between 1.4x and 2.6x (Area B, FIG. 2), spontaneously crimps to at least five helical crimps per inch. Further, the yarns drawn between 1.8x and 2.2x had the highest number of crimps per inch, namely, about 50. On the other hand, the yarns drawn more than 2.6x (Area C, FIG. 2) either did not crimp at all or had, at best, one to three crimps per inch, too little to be of any utility in yarn processing. The crimped fibers are of good quality with tenacities varying from 2.2 to 3.7 grams per denier, depending on the draw ratio used.

Referring now to the straight line curve identified as 25 PSI. Steam, FIG. 2, this is based on a series of points determined by drawing the above as-spun yarn through a steam cell containing saturated steam at 25 psi. gauge rather than around a heated draw pin, at various draw speeds ranging up to 500 y.p.m. It will be noted that even at a draw speed of 500 y.p.m. the maximum draw ratio was above 3.2x. The various samples of drawn yarn here are all drawn well above 2.6 X and, in fact, the whole curve is in Area C. None of these yarn samples shows any spontaneous crimp either upon release of the drawing temperature or even after relaxing in a 130 C. oven. This again illustrates the importance of the 2.6x upper limitation on the draw ratio in the drawing step of this invention when using, as the starting yarn, one having gamma orientation and an orientation angle of 10 to 36. Yarn not having these requisite characteristics cannot be made to crimp by any modification of this process so far as known.

The significance of the draw ratio is further demonstrated by preparing a starting polypropylene yarn in the same manner as above by using a polypropylene of melt index 22 (inherent viscosity of 1.5). This polymer is made by heating the aforementioned 0.7 melt index polypropylene in an extruder at 230 C. in the presence of tbutyl hydroperoxide and then adding 0.1% 4,4'-butylidene bis(6-t-butyl m-cresol) as a stabilizer. This yarn has a maximum draw ratio on a 25 C. pin of 6.4 and .4 at 70 and 250 y.p.m., respectively. As expected the use of a higher temperature on the draw pin or the use of the steam cell results in even higher maximum draw ratios. Samples of this yarn drawn at these maximum draw ratios, all significantly above 2.6 do not spontaneously crimp when tension is relieved or even after relaxing in boiling water or heating in an air oven at 130 C. for 10 minutes.

EXAMPLE IV This example illustrates that the lower limit of 1.4x on the draw ratio in the drawing step of this invention is important in order to obtain a yarn having a significant number of crimps per inch. That is, a yarn having a maximum draw ratio below 1.4 (Area A, FIG. 2) under the conditions prevailing or one drawn less than 1.4x even though it may have a higher maximum draw ratio, is not crimped satisfactorily regardless of whether or not the other conditions of the process of this invention are observed. The example also shows a draw ratio of about 2 gives maximum crimps per inch.

The as-spun yarn of Example III made from the 0.7 melt index polypropylene is drawn at the draw ratios tabulated in Table II below to give yarn samples as reported in Table II. In group A, the yarn in each instance was drawn over a 75 C. pin at a draw speed of 650 y.p.m.; in group B, the yarn in each instance was likewise drawn over a 75 C. pin but at a draw speed of 240 y.p.m.

8 Table II Draw ratio Irodu ct,

Actual Maximum EXAMPLE V Table III Percent of max. draw ratio Draw ratio Crimp/Inch Crimp index Here, as in Example IV, the use of a draw ratio under 1.4x provided too little crimp for practical significance.

In both examples the maximum crimp effect is obtained using a draw ratio in the neighborhood of 2X. The draw ratio that will give the maximum crimp effect, will vary somewhat from one instance to the next but will normally lie between 1.8x and 2.2x which is the preferred range for the draw ratio.

EXAMPLE VI A bobbin of the drawn yarn of Example I that has been kept taut on the package, is immersed in a 2% solution of the dye C.I. acid red 15,675 in a 50% aqueous ethanol at C. for 2 minutes. The yarn is dyed a deep shade of red that is wash-fast. When the thus-dyed yarn is permitted to relax, it spontaneously crimps.

The above yarn may also be dyed by placing it in the above dye bath in an untensioned condition. How ever, the yarn must not be in an untensioned (e.g., off the bobbin state) for more than one minute at 30 C. or more than 2 weeks at 5 C. if it is to be dyed to the deepest shades by this procedure. After about 24 hours in an untensioned condition at 30 C., the fiber is not even stained by the above dye bath.

In a similar manner solutions of the dye C.I. 64,500 (C.I. disperse blue 1) in aqueous ethanol (ZS-99%), ethanol, N,N-dimethyl formamide and methylene chloride are used to dye the above unrelaxed yarn to deep blue shades. The method works with all types of dyes dispersed, acidic, or basic but only solutions-not dispersions-of the dyes are effective.

The mechanism of the dyeing is not understood but most polypropylene yarns drawn under the conditions of this invention to aiford a spontaneous helical crimp and dyed before releasing tension or under the special conditions above, display this unique dyeability to a remarkable extent. On the other hand, yarns that do not crimp, do not dye by this method or to only light shades.

EXAMPLE VII This example illustrates the modified process of this invention wherein the polypropylene filament, after drawing but before tension is released, is passed at least 30 around a hard surface having a radius of curvature less than 0.5 inch. In this modified process a draw ratio only 25% of the maximum draw ratio is required. Actually, a draw ratio of 50% of the maximum draw ratio is illustrated in the example.

The example also illustrates the critical nature of the angle of orientation of the starting filament in getting a satisfactory crimp according to this invention. Where the angle of orientation appreciably exceeds 36, the resulting crimp, if any, is not satisfactory. Further, the example illustrates how a filament having an unsuitably higher angle of orientation, may be conditioned by heating while relaxed, to reduce the angle of orientation and thus obtain a filament admirably adapted for use as the starting mate rial in the present invention.

Table IV sets forth data on various as-spun polypropylene filaments that have not been subjected to any preliminary drawing. In each instance, the as-spun filament is drawn 1.5 X in room temperature air, the maximum draw ratio under such conditions being 3x, and, while still under tension, passed 90 round an unheated 0.25 inch diameter metal pin. The items in Table IV are reported in pairs, the first filament of a pair not being given any heat treatment prior to drawing and the second of a pair being the same filament but having been heated in a 130 C. oven in a tensionless condition for minutes prior to drawing; this heat treatment is designated Hot Relaxed in Table IV.

1 195 as measured on filament. 2 Gamma intensity ratio.

The first pair of filaments (A-1 and A-2) reported above typically show how radically the angle of orientation can be reduced (from 43 to 27) by the heat treatment and what a drastic difference in result is obtained by the present process depending upon the angle of orientation of the starting filament. Items B, C, and D likewise illustrate this. Item B also illustrates that the heat treatment iseffective to convert a filament exhibiting potential gamma orientation to one having the gamma orientation called for in the starting filament for use in this invention.

Item E in Table IV illustrates the relatively mild infiuence of the heat treatment applied to a filament having an angle of orientation well below 36. The actual crimp measured is greater in E-l which was not subjected to the heat treatment, than in E-2 which was, although it is not contended that any important advantage is obtained in the crimp by omitting the heat treatment in this instance.

It will be understood that the above examples are merely illustrative and the present invention broadly comprises drawing a polypropylene filament having gamma 10 orientation and an orientation angle of 10 to 36, such filament not having been pre-drawn at a draw ratio of greater than 1.86X, at a draw ratio of at least 1.4x but not above a total draw ratio, including any pre-drawing, of 2.6x, under conditions that the total draw ratio is at least 60% of the maximum draw ratio, and thereafter releasing the drawing tension whereby the filament develops at least five helical crimps per inch. A further step may be included, after the filament has been drawn, of passing the drawn filament under tension around a hard surface having a radius of curvature less than about 0.5 inch and then releasing the tension; in such case, the filament need only be drawn 25% of the maximum draw ratio rather than at least 60%.

As previously noted, FIG. 3 is a graphic representation of an X-ray diffraction photograph of a polypropylene filament having gamma orientation and being suitable for use in this invention. For purposes of contrast, FIG. 4 is a similar representation of a polypropylene filament having normal rather than gamma orientation which filament is not adapted for use in the present invention.

The difiractions 21 located on the circle 22 (26:14.0) at an azimuthal angle greater than 50 from the equator 23 in FIG. 3 are characteristic of gamma orientation. No such diffraction is observed on the circle 22 of FIG. 4. The portion of the diifraction 24 that is centered on circle 25 (20=16.8) determines the orientation angle of the filament. The higher crystallite orientation angle of FIG. 3 (16) as compared to FIG. 4 (13) is apparent. The difir .ctions 26 are typical of all oriented polypropyh ene filament but are not pertinent here. The diffractions 27 are located on a circle 2% (20:28.6) and are used in the determination of gamma intensity ratio. It should be noted that the pattern is symmetrically divided by the equator. The meridian is located at right angles to the equator through the center of the pattern.

To those skilled in the art, the diiference in the difiraeti-on patterns of FIGS. 3 and 4 is striking and. significant. The diifr-actions 21 of FIG. 3 are unusual and immediately reflect the unique crystalline structure of the polypropylene filaments suitable for use in the present invention.

So far as known, Australian application 36,834, dated October 4, 1958, appears to be the closest prior art to this invention. It purports to disclose a process whereby polypropylene filaments are crimped without resorting to mechanical means. As shown, the crimps are highly irregular, obviously neither similar to the zigzag crimp resulting from mechanical crimping or the helicals crirnp of the instant invention. The process of the Australian application features a drawing of the filaments 2.5 X to 5.5x and then bulking the drawn filaments by heating them in the form of a skein. Attempts to carry out the examples of this reference to reproduce the results described therein have not been successful, even when restricting the draw to 2.5 X. Of course, when starting with filaments having the characteristics called for by the instant invention and drawing them 2.5 X, the helical crimp of this invention is obtained and without any heating step. But the Australian application does not disclose using such filaments as the starting material.

The fact is the disclosure of the above-mentioned Australian application is substantially completely silent as to conditions for spinning the yarn and, depending upon the conditions used, filaments of widely differing characteristics obviously can be obtained. It is believed failure to reproduce the results described in this reference is due to the use of spinning conditions diiferent from those actually used (but not disclosed) in the examples of the Australian reference. No contention is made that it is impossible to obtain the results described in the Australian reference when filaments are treated as specified in the examples of the reference providing conditions are used to give, as the starting material, a filament of the proper characteristics. The difliculty lies in the failure of the reference to disclose what specific conditions were used in the spinning. However, it is quite clear that the starting filaments used in the Australian reference must have been radically different from those specified herein as otherwise the Australian procedure would have resulted in a spontaneous helical crimp without employing any heat treatment of the relaxed filaments if the draw ratio did not appreciably exceed 2.6x, or the filaments would not have crimped at all even after the heating treatment if the draw ratio had been in the upper portion of the disclosed range of 2.5 X to 5.5 X. Actually, Examples 1 and 2 of the Australian application use draw ratios of 4x and 3.5x, respectively, and at such high draw ratios no crimping at all results using the starting filaments of the present invention, regardless of whether the added heating step of the Australian reference is used or not.

The present process is notably advantageous in that it is simple and economical to carry out, particularly as contrasted to one involving crimping filaments by mechanical means, and, at the same time, it results in a helically crimped filament of unimpaired physical properties. The filaments made by the instant process are suitable in general for the known uses of crimped polypropylene filaments and are especially adapted for use in fabrics of exceptional aesthetic appeal, both because the crimp is of helical design and because the filaments can be effectively dyed as disclosed herein.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

The invention claimed is:

1. Process of preparing a helically crimped polypropylene filament which process comprises drawing a polypropylene filament having gamma orientation and an orientation angle of to 36, said filament not having been pre-drawn at a draw ratio of greater than 1.86 at a draw ratio of at least 1.4x but not above a total draw ratio, including any pre-drawing, of 2.6x, under conditions that said draw ratio is 60% to 100% of the maximum draw ratio, and thereafter releasing the drawing tension whereby said filament develops at least five helical crimps per inch.

2. The process set forth in claim 1 wherein said polypropylene filament used as the stanting material has a gamma intensity ratio of at least 1.0 and an orientation angle of 10 to 30.

3. The process set forth in claim 1 wherein said polypropylene filament is passed around a snubbing surface during said drawing step.

4. The process set forth in claim 1 wherein said polypropylene filament is subjected to a solution of dye subsequent to said drawing step and not appreciably over a minute after releasing said drawing tension.

-5. Process of preparing a helically crimped polypropylene filament which process comprises drawing a polypropylene filament having gamma orientation and an orientation angle of 10 to 36, said filament not having been pre-drawn at a draw ratio greater than 1.2x, at a draw ratio of at least 1.8x but not above a total draw ratio, including any pre-drawing of 2.2x, under condi- 12 rtions that said draw ratio is 70% to 100% of the maximum draw ratio, and thereafter releasing the drawing tension whereby said filament develops at least five helical crimps per inch.

6. A process of preparing a helically crimped polypropylene filament which process comprises drawing an undrawn polypropylene filament having gamma orientation and an orientation angle of 10 to 36, at a draw ratio of at least 1.4x but not above 2.6x, under conditions that said draw ratio is to 100% of the maximum draw ratio, and thereafter releasing the drawing tension whereby said filament develops at least five helical crimps per inch.

7. The process set forth in claim 6 wherein a draw ratio of 1.8x to 2.2x is used under conditions such that said draw ratio is to of the maximum draw ratio.

8. Process of preparing a helically crimped polypropylene filament which process comprises drawing an undrawn polypropylene filament having gamma orientation, a gamma intensity ratio of at least 1.0, and an orientation angle of 10 to 30, at a draw ratio of 1.8x to 2.2x under conditions such that said draw ratio is 70% to 100% of the maximum draw ratio, passing said filament around a snubbing surface during said drawing step, subjecting said filament to a solution of dye subsequent to said drawing step and not appreciably over a minute after releasing the drawing tension, and thereafter releasing said drawing tension whereby said filament develops at least five helical crimps per inch.

9. Process of preparing a helically crimped polypropylene filament which process comprises drawing a polypropylene filament having gamma orientation and an orientation angle of 10 to 36, said filament not having been pro-drawn at a draw ratio of greater than 1.86X, at a draw ratio of at least 1.4x but not above a total draw ratio, including any pre-drawing, of 2.6x, under conditions such that said draw ratio is at least 25% of the maximum draw ratio, passing said drawn filament under tension at least 30 around a hard surface having a radius of curvature less than 0.5 inch, and thereafter releasing the tension whereby said filament develops at least ten helical crimps per inch.

'10. Process of preparing a helically crimped polypropylene filament which process comprises drawing an undrawn polypropylene filament having a gamma intensity ratio of at least 1 and an orientation angle of from 10 to 30, at a draw ratio of at least 1.4x but not above 2.6 x, under conditions such that said draw ratio is 50% to 100% of the maximum draw ratio, passing said drawn filament under tension at least 30 around a hard surface having a radius of curvature less than 0.5 inch, and thereafter releasing the tension whereby said filament develops at least 10 helical crimps per inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,289,232 Babcock July 7, 1942 2,925,641 Evans Feb. 23, 1960 FOREIGN PATENTS 220,947 Australia Mar. 24, 1959 

1. PROCESS OF PREPARING A HELICALLY CRIMPED POLYPROPYLENE FILAMENT WHICH PROCESS COMPRISES DRAWING A POLYPROPYLENE FILAMENT HAVING GAMMA ORIENTATION AND AN ORIENTATION ANGLE OF 10* TO 36*, SAID FILAMENT NOT HAVING BEEN PRE-DRAWN AT A DRAW RATIO OF GREATER THAN 1.86X, AT A DRAW RATIO OF AT LEAST 1.4X BUT NOT ABOVE A TOTAL DRAW RATIO, INCLUDING ANY PRE-DRAWING, OF 2.6X, UNDER CONDITIONS THAT SAID DRAW RATIO IS 60% TO 100% OF THE MAXIMUM DRAW RATIO, AND THEREAFTER RELEASING THE DRAWING TENSION WHEREBY SAID FILAMENT DEVELOPS AT LEAST FIVE HELICAL CRIMPS PER INCH. 